The history of the onna-bugeisha, literally meaning “woman warrior,” can be traced back to as early as 200 AD, when Empress Jingū, following the death of her husband Emperor Chūai, took to the throne and led an invasion of Silla (modern day Korea).

And:

DNA tests on 105 bodies excavated from the Battle of Senbon Matsubaru between Takeda Katsuyori and Hojo Ujinao in 1580 revealed that 35 of them were women. According to Turnbull, the details on the excavation confirm that women warriors were almost certainly present on the battlefield.

This is a fascinating article, with excellent pictures from Wikimedia Commons, like this one of Tomoe Gozen. In 1184, she led 300 samurai into a fierce battle against 2,000 opposing Tiara clan warriors.